Tidal and Wave Energy: The Resource That Keeps a Timetable
The ocean presents an unusual case in energy: two technologies with genuine, specific advantages that almost every other renewable lacks, and a deployment record that remains close to nothing. Understanding why is more instructive than either the enthusiasm or the dismissal, because the obstacle is not the one most people assume.
Two Resources, Often Confused
Tidal energy derives from the gravitational interaction of the Earth, Moon and Sun. Twice a day the ocean rises and falls, and where coastal geography funnels that movement - a narrow strait, a long estuary - the water moves fast enough to drive a turbine. The energy ultimately comes from the Earth's rotation, which tidal friction very slowly slows.
Wave energy is unrelated in origin. Wind blowing across water transfers momentum to the surface, and the resulting waves carry that energy sometimes thousands of kilometres from where the wind blew. A wave arriving in Ireland may have been generated by a storm off Newfoundland days earlier. Wave energy is therefore concentrated wind energy, with the ocean acting as both collector and store.
The practical consequences differ completely. Tides are utterly predictable but fixed in timing, so a tidal plant generates when the tide says and not when demand does - though the schedule is known and can be planned around, unlike wind or solar. Waves are less predictable but forecastable several days out, and they persist after the wind drops, which makes wave output smoother than wind output at the same site.
Water density is what makes both attractive. Seawater is roughly 830 times denser than air, so a given flow speed carries far more energy. A two-metre-per-second tidal current has comparable power density to a wind speed of around 18 metres per second, which is a gale. That is why tidal turbines are small compared with wind turbines of the same rating.
How Each Is Captured
Tidal range schemes are the older approach: build a barrage across an estuary, let the tide fill the basin, and release it through turbines at low tide. La Rance in France has operated since 1966 at 240 megawatts, and Sihwa Lake in South Korea at 254 megawatts since 2011. They work, they last, and they flood an estuary - which is why very few have been built since, environmental objections having become decisive.
Tidal stream schemes place turbines directly in a fast current, without damming anything. They resemble underwater wind turbines and have become the main line of development. MeyGen in the Pentland Firth off Scotland is the largest, and individual machines now exceed two megawatts. Because water is dense, a tidal turbine of 20 metres diameter matches a wind turbine of 80.
Wave devices have not converged on a single form, which is itself informative. Point absorbers bob vertically and drive a generator through the motion. Attenuators are long articulated structures that flex as waves pass along them. Oscillating water columns use a wave to push air through a turbine in a chamber. Overtopping devices let waves spill into a raised reservoir and drain back through a low-head turbine.
Wind converged on the three-bladed horizontal-axis turbine by the 1990s and has barely deviated since. Wave energy has not converged after fifty years, which suggests no configuration has yet proved clearly superior - a meaningful signal about the state of the field.
The Predictability Advantage
Tidal generation can be forecast with high accuracy decades ahead, because it depends on orbital mechanics. Tide tables for 2050 can be printed today and will be correct. No weather-dependent source offers anything comparable; wind and solar forecasting has improved greatly but remains probabilistic hours to days out.
For a grid operator this is worth more than the raw energy suggests. Scheduled, known output reduces the reserve margin that must be held, and it can be built into a plan rather than accommodated as it arrives. It is a different property from the continuous availability of geothermal or nuclear fission - tidal output still goes to zero four times a day - but predictable variability is far easier to manage than unpredictable variability.
Geography helps further. High tide occurs at different times along a coastline, so a portfolio of tidal sites in different locations can produce a much flatter combined output than any single site. Studies of the United Kingdom, which holds a large share of Europe's tidal resource, suggest a distributed set of sites could supply a near-continuous baseline.
Wave energy offers a different systemic benefit: it correlates poorly with solar and only partially with local wind, and it is strongest in winter in the North Atlantic, when solar is weakest. A resource that peaks when the others do not reduces the storage a system needs, which is worth more than a comparison of energy costs alone would suggest.
Why Almost Nothing Is Built
Global installed ocean energy capacity is roughly 500 megawatts, most of it the two old tidal barrages. Wind exceeds 1,000 gigawatts. The ratio is about two thousand to one, and it has not moved much in a decade despite sustained public funding in several countries.
The reason is not the resource, which is large and well mapped. It is that the ocean destroys equipment. Seawater corrodes metal and degrades seals continuously. Marine growth accumulates on any surface within months. And the design case is not the average condition but the hundred-year storm, where forces can exceed normal operating loads by a factor of ten or more - so a device must be built for a day that may never come during its service life, and that expense is carried by every kilowatt-hour it produces.
Maintenance compounds it. A wind turbine on land is reached by a van. A wave device twenty kilometres offshore needs a specialist vessel, a weather window and a crew, and a fault in November may wait until March. Several leading developers have failed after devices survived technically but proved uneconomic to keep running.
The honest assessment is that ocean energy today is where offshore wind was around 1995: demonstrably functional, far too expensive, and dependent on whether an industry forms to drive the learning curve. Offshore wind fell about 70 percent in cost once deployment reached scale. Whether tidal and wave follow depends less on any technical breakthrough than on whether enough units are built for the same learning to occur - and that is a question about industrial policy rather than about the sea.
Frequently asked questions
What is the difference between tidal and wave energy?
Tidal energy comes from the gravitational pull of the Moon and Sun moving water horizontally on a fixed schedule. Wave energy is wind energy transferred to the sea surface, which can travel thousands of kilometres from where the wind blew. They share only the medium; their predictability, equipment and economics are entirely different.
Why is tidal energy predictable?
Because it depends on orbital mechanics rather than weather. The positions of the Moon and Sun are known with great precision far into the future, so tide tables for decades ahead can be produced today and will be accurate. No weather-dependent renewable has that property.
Why is ocean energy so much denser than wind?
Seawater is about 830 times denser than air, so the same flow speed carries far more energy. A tidal current of two metres per second has power density comparable to a wind speed of about 18 metres per second. That is why a 20-metre tidal turbine matches an 80-metre wind turbine.
If the resource is so good, why is so little built?
Survivability and cost. Seawater corrodes, marine growth accumulates, and devices must be engineered for hundred-year storms with forces ten times normal operation. Maintenance requires a vessel and a weather window. Global capacity is around 500 megawatts against more than 1,000 gigawatts of wind.
Will tidal and wave energy ever become significant?
It depends on deployment volume rather than on a technical breakthrough. Offshore wind fell roughly 70 percent in cost once enough units were built to drive a learning curve. Ocean energy today resembles offshore wind around 1995 - functional, expensive, and waiting to see whether an industry forms at sufficient scale.
